Project Summary/Abstract For many low-income persons aged 65 and older, both Medicare and Medicaid can have significant impacts on access to healthcare, healthcare spending, and overall health. Medicare serves as the primary payer of their healthcare, while Medicaid is intended to cover cost-sharing and services that Medicare does not. However, three important connections between these two programs are poorly understood. First, although past estimates suggest that participation rates are very low, up-to-date estimates of Medicaid program participation among adults aged 65 and older are unavailable. Second, the factors associated with Medicaid participation by seniors are also understudied. Third, the extent to which recent changes in the Medicaid program have spillover effects on low-income seniors' healthcare use, spending, and health is unknown. The broad objective of this proposal is to better understand linkages between Medicare and Medicaid in the form of dual program enrollment and indirect spillover effects of Medicaid program expansions on community-dwelling low-income seniors who are enrolled in Medicare and eligible for full Medicaid benefits, hereafter referred to as low-income seniors. This project has three specific aims: 1) to conduct a comprehensive analysis of full Medicaid program participation by low-income seniors to identify differences in Medicaid program participation over time and across subgroups and precipitating factors associated with program participation; 2) to investigate the extent to which recent policy changes under the Affordable Care Act lead low-income seniors to enroll in the program; and 3) to leverage recent policy changes to identify the net spillover effects of Medicaid expansions to the non- elderly on health and healthcare use among low-income seniors. This project will first compile a detailed dataset of full Medicaid eligibility criteria for all U.S. states and for each year from 2006-2020. Those eligibility criteria will be used to identify low-income seniors eligible for full Medicaid in several existing large publicly available and restricted use national household and individual surveys. These surveys complement each other in terms of their scope, focus, and sample population. This project will produce estimates of the rates of full Medicaid participation by low-income seniors over time and for various subgroups, and will identify life events that immediately precede full Medicaid enrollment. Finally, this project will use quasi-experimental methods and multivariate regression analysis to estimate the unintended spillover effects of policies that expand Medicaid coverage to non-elderly adults on Medicaid participation, healthcare utilization, spending, and health status among low-income seniors. This project will provide important evidence that will inform strategies to address the health needs of the high-cost, vulnerable population of low-income seniors. The project will also inform policies to increase Medicaid participation and will identify potential unintended consequences of the ACA's Medicaid expansions to the non-elderly population in the form of changes in access and health among low-income seniors.